Electrical apparatus



June 2 1932' w, s, PURSER 1,865,210

' ELECTRICAL APPARATUS Filed July 25. 1928 INVENTOR Wmum SPENCERP11125512 Patented June 28, 1932 TENT FFlE

WILLIAM SPENCER PURSER, OF LONDON, ENGLAND, ASSIG-NOR TO COLUMBIAPHONE;-

GRAIH COMPANY, INC., 033 BBIDGEEORT, CONNECTICUT, A CORPORATION OF NEWYORK ELECTRICAL APPARATUS Application filed July 25, 1928, Serial No.295,336, and. in Great Britain September 9, 1927.

This invention relates to microphone transmitters of the kind embodyinga diaphragm and carbon granules such as is employed in connection withthe electrical recording of sounds, wireless telephony, public addressinstallations and for other like purposes.

The principal object of the invention is to provide a microphone whichwill be responsive to a much wider range of frequencies to than thosehitherto constructed and which will give a substantially equal responseover the entire audible frequency range with which the microphone willbe called on to deal.

A further object of the invention is to increase the sensiriveness ofthe microphone and to eliminate or minimize the hissing noise commonlyexperienced with carbon rai'iule microphones as hitherto produced.

A still further object is to reduce or eliminate the rattling sounds dueto looseness of the granules at the top of the microphone whichsometimes occurs owing to age.

The invention consists in a microphone of the kind referred to having adiaphragm of substantially triangular shape.

The invention also consists in a microphone of the above characterhaving a carbon granule chamber of substantially triangular shape withthe apex at the top.

The invention further consists in a microphone of the above characterhaving two electrodes, the one arranged at or approximately at the apexof the triangle and the other at or approximately at the middle point ofthe base.

The invention also consists in other details and arrangementshereinafter described or indicated.

The accompanying drawing illustrates one convenient form of microphonein accordance with the invention.

Fig. 1 is a front elevation; and

Fig. 2 is a side sectional elevation.

In carrying my invention into effect in one convenient manner I form myimproved microphone with a circular or other shaped retaining case 5which may be made of metal or of any other suitable material but whichis preferably relatively heavy. l Vithin the Cit . casing is a recess 6lined with some insulating lining 7 such as ebonite and of substantiallytriangular form with the apex arranged at the top, the recess beingprovided with gaskets 8 on which is supported the diaphragm 9 which isformed of suitable material and which is of a shape similar to that ofthe recess, that is to say, of substantial triangular form.

lVithin the carbon granule recess I arrange two electrodes whichconveniently take the form of polished carbon microphone but tons 10,one of such electrodes being arranged at the apex of the triangle whilethe other is at the centre of the base so that a line drawn through thecentres of the buttons would form a rightangle with the base line of thetriangle as clearly seen from Fig. 1.

Connection to the electrodes is effected by means of insulated wirespassing through the container to terminals 11 at the baca of themicrophone while a cover 12 of suitable form may be fitted in front ofthe diaphragm to prevent the same being accidentally damaged.

The triangular shape of the granule recess results in the prevention ofthe packing of the carbon granules which has hitherto been anundesirable feature of carbon granule microphones and which is due tothe weight of carbon in the top half of the granule container bearingupon the granules in the lower part.

Owing to the triangular shape in accordance with the present invention,the weight of carbon granules in the top half of the cha1nher is verymuch less than that in the lower half so that such packing is therebyavoided or reduced to a minimum. Should, however, any slight tendency topack occur due to a blow or to excessive mechanical vibration, theelectrical disadvantages due to this are avoided by the disposition ofthe polished carbon electrodes since any packing of the lower half ofthe carbon chamber will be compensated by a corresponding loosening inthe upper half so that the feed current of the microphone will remainconstant.

A further means of avoiding packing and of blast also is by keeping thecarbon granule chamber as small as possible and by recessing the carbonelectrodes slightly in the in thickness of diaphragm be chosen such thatV a small strip of a length equal to the length of the 'base' willresonate "atr a point where the higher frequencies normally commence .to

fall off in power considerably,itwill be seen that each strip of thediaphragm will resonate from this frequency right up to the limit ofaudibility. Moreover, since the diaphragm is triangular in shape and isrigidly "supported on each side, the diaphragm as a whole will have noresonance of its own.

The sides of the diaphragm and of the carbon granule chamber arepreferably'straight but may be slightly curved if desired.

In the case of carbon granule microphones,

it has been found thatowing to'age, the carbon granules settle downandoccupy less space after a time than they did'when first putinto themicrophone. This results in a small air space between the top of thegranules and the filling hole plug, and gives rise to rattling soundsdue to looseness' of the granules at the top of the microphone. I Inorder to overcome this, a plug 13fitted 'with a spring plunger 14 maybe'provided of such dimensions and strength of spring that V there willbe always the'same tension'or pressure on the granules, and any settlingdown will be taken up by'thespr1ng,thus preventing the rattling.

The inventlon 18 not to be limited to" the constructional detailshereinbefore given by way of example, since these may be modified tosuit any particular apparatus for which the microphone is to'be employedor any practical requirements that may have to be ful- "and inwhatmanner thesame is to be per- Having now particularly described andascertalned the nature of my said invention formed, I declare that whatI claim is 1. In a microphone, a body portion, a substantiallytriangular-shaped carbon granule chamber within said body portion, twoelectrodes within said carbon granule chamber, and one of saidelectrodes being arranged approximately at the apex of the triangle andthe other of said electrodes being arranged approximately at the middlepoint of the base of the triangle. I

2. In a microphone, a body portion, a substantiallytriangular shapedcarbon granule chamber within saidbody portion, carbon granules in saidchamber, a plunger extending into said chamber through the apex thereof,and a spring normally acting to urge the plunger against the granules toprevent rattling thereof in the chamber.

Signed at London, England, this 11th day :of'July, A. I). 1928.

WILLIAM SPENCER PURSER.

